Electroluminescent device with integral arc-suppressing means



Sept. 10, 1963 E. J. HAM 3,103,608

ELECTROLUMINESCENT DEVICE WITH INTEGRAL ARC-SUPPRESSING MEANS Filed Nov. 9, 1961 FIG. I.

VOLTAGE 1L sounce FIG. 2.

V// I I/ l l '3 B \2 l4 l6 l8 INVENTOR.

EDM/HED I HHM.

United States Patent ELECTROLUMINESCENT DEVICE WITH INTE- GRAL ARC-SUPPRESSING lVlEANS Edward J. Ham, Rockaway Township, Morris County,

NJ., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 9, 1961, Scr. No. 151,258 6 Claims. (Cl. 313-408) trode and a glass plate which is coated with a transparent conductive coating, such as tin oxide or the like, that serves as the second electrode. In order to facilitate manufacture, the metal electrode is customarily formed over the phosphor-dielectric layer by vacuum-deposition techniques so that the metal electrode is inherently very thin. Since the light output of electroluminescent phosphors depreciates rapidly in the presence of water vapor a protective coating of a suitable moisture-impervious material such as epoxy resin is applied to the backand sides of the device to seal it oif'from the atmosphere. An electroluminescent device encapsulated with epoxy is described in detail in copending application Serial No. 822,231 of W. J. Knochel et al., filed June 23, 1959, now. Patent No. 3,030,542, issued April 17, 1962, and assigned to the assignee of the present invention.

The extreme thinness of the vaporized metal electrode employed in such glass-plastic electroluminescent devices presents a serious problem in that the metal electrode is very susceptible to damage from electric arcs. Thus,

when lamps of this construction are subjected to a voltage that exceeds the break-down voltage of the phosphordielectric layer the resultant electric arc ruptures the thin metal electrode leaving a burnt out area or blemish that is visible through the glass face plate. Once the arc punctures the metal electrode and reaches the flammable epoxy coating it may spread over a much larger area and thus greatly increase the burnt out are-a. When this occurs the resulting large blemishes or lines produced by the arc renders the lamp useless from both an appearance and operational standpoint. In some cases, the are becomes so intense that it cracks the lamp thus creating a potential safety hazard.

It is accordingly the general object of the present invention to provide an improved plastic-backed electroluminescent device that has a longer useful life than the prior art devices having such a protective backing.

Another and more specific object is the provision of a simple and inexpensive means for suppressing arcs within an electroluminescent device, particularly those of glass-plastic construction, and thus minimizing the damage resulting from such arcs.

The foregoing objects, and others which will become obvious to those skilled in the art as the description proceeds, are achieved in accordance with this invention by placing a sheet of nominflammable dielectric material between the thin metal electrode and the protective plastic coating. More specifically, a thin sheet of mica is bonded to the thin metal electrode by a film of Canada balsam before the epoxy coating is applied to the partly finished lamp. The resulting combination of mica and the cured Canada balsam film provides a two-ply insulating barrier between the metal electrode and epoxy coating that is particularly effective in quickly quenching electric arcs and minimizing the damage to and blemishing of the lamp.

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A better understanding of the invention will be ob- FIG. 1 is a perspective view of Ian electroluminescent device embodying the arc-suppressing barrier of this invention; and,

FIG. 2 is a cross-sectional view through the device shown in FIG. 1 along the line II--II, in the direction of the arrows.

While the present invention may be advantageously employed in electroluminescent devices of various types and construction, it is particularly adapted for use in glass-plastic type electroluminescent lamps and has accordingly been so illustrated and will be so described.

In FIG. 1 there is shown an electroluminescent lamp 10 of the so-called glass-plastic type in which the well known electrode-phosphor cell or element is supported by a vitreous base member such as a glass face plate 12. A protective coating 24 of a suitable thermosetting moisture-impervious plastic suchas epoxy is applied over the back and sides of the lamp and is hermetically bonded to the peripheral edges 13 of the glass plate 12 (see FIG. 2) thus forming a protective casing and sealing 01f. the light-generating core of the lamp from the atmosphere. A plastic-encapsulated electroluminescent lamp of this type is disclosed and claimed in the above-mentioned copending application Serial No. 822,231 of W. J. Knochel et al. A pair of lead-in conductors 25 and 26 aresealed through the plastic coating 24 and are conneotcd to the lamp electrodes in the usual manner to enablethe'lamp to be operated from a suitable voltage source (not shown). I

As is illustrated in FIG. 2, the electroluminescent lamp 10 in accordance with this invention consists of the following components, superimposed one over the other in the order named; a glass face plate 12 that transmits radiations, a radiation-transmitting electrode 14 such as tin oxide or the like; a phosphor-dielectric layer 16 consisting of a suitable electroluminescent phosphor, such as copper-activated zinc sulfide, embedded in a suitable plastic dielectric material such as polyvinyl-chloride acetate, a thin metal electrode 118 such as vaporized aluminum or the like, a thin film of Canada balsam 20, a sheet 22 of suitable non-inflammable dielectric material such as mica, and a coating 24- of cured thermosetting plastic such as epoxy resin. As shown, the film of Canada balsam 20 and the mica sheet 22 are coextensive with the thin metal electrode 18, the Canada balsam preferably extending over and around the edges of the metal electrode. The epoxy coating 24 also overlaps the edges of the mica sheet, metal electrode and phosphor-dielectric layer and is hermetically bonded to the glass plate 12 so as to prevent the ingress of Water vapor or other deteriorating gaseous impurities.

The mica sheet 22 is preferably bonded to the Canada balsam film 20 to provide an hermetic seal between these components. It has been found that a satisfactory bond between the Canada balsam film and the mica sheet can be obtained during the fabrication of the lamp before the plastic overcoat is applied by brushing or spraying the Canada balsam lacquer onto the metal electrode 18, allowing it to cure, and then placing the mica sheet over the cured lacquer and heating the lamp in an oven to a temperature of about 00 C. for approximately five minutes. This heat treatment causes the Canada balsam to soften slightly and wet the mica sheet. Upon removal from the oven the Canada balsam hardens producing the desired bond. Canada balsam is a turpentine that is obtained from balsam fir trees and is a yellowish liquid that becomes transparent when it solidifies.

Since the epoxy coating 24 is applied to the lamp while 3,. in the liquid state and then cured in situ, it inherently is bonded to the mica sheet 22. Epoxy resins are well known condensation products of epichlorohydrin and diphenols, such as bisphenol A, and are particularly suited for use as the protective overcoat since they are thermosetting,. moisture-impervious and have low cure shrinkage and,

high strength.

As a specific example of the relative dimensions of the various parts, excellent results have been obtained by employing a mica sheet about '1 mil thick and a film of Canada balsam about 0.5 mil thick in anelectrolum'inescent lamp having a phosphor-dielectric layer about 1 mil thick, an aluminum electrode about 0.02. mil thick and an epoxy backing approximately 35 mils thick.

The inclusion of an insulating barrier of fireproof material between the Canada balsam film and protective plastic coating in accordance with the present invention prevents any arcs which may occur between the electrodes from reaching the plastic backing. Thus, even though an arc may puncture the thin metal electrode 18 it is prevented from spreading and is thus eifectively controlled.

In addition, the Canada balsam appears to have a quenching eflect on electric arcs and this property seems to be enhanced by the presence of the insulating sheet of mica for some reason not yet perfectly understood. This quenching effect, in conjunction with the isolation of the epoxy coating from the arc itself achieved by the insulating barrier, quickly extinguishes any arcs that may occur and minimizes the resultant damage to and blemishing of the lamp.

Comparative tests have shown that lamps made with an arc-suppressing and insulating barrier in accordance with this invention withstand voltage surges that greatly exceed the breakdown voltage of the lamp with very few arcs occurring compared to conventional lamps under the same surge conditions. Moreover, the blemishes resulting from the arcing that did take place in the improved lamps were of pin point character and not objectionable from either an appearance or functional standpoint indicating that the integrity of the electrode-phosphor core of the lamp was substantially unimpaired even after such rigorous testing. 7

It will be appreciated from the foregoing that the objects of the invention have been achieved by providing a plastic-backed electroluminescent lamp that is less susceptible to damage from internal arcs and which has a longer useful life than prior art lamps of this type.

While one embodiment of the invention has been illustrated and described in detail, it will be apparent that various modifications in the organization of parts and selection of materials may be made without departing from the spirit or scope of this invention.

I claim:

1. In an electroluminescent device, the combination of an electrode that is susceptible to damage from electric arcs, a layer of Canada balsam on the outer surface of said electrode, a sheet of dielectric material over said layer of Canada balsam, and a coating of cured thermosetting plastic covering said sheet of dielectric material.

2.. The combination set forth in claim 1 wherein, said electrode comprises a thin layer of vaporized metal, and said layer of Canada balsam is bonded to said sheet of dielectric material.

3. The combination set forth in claim 1 wherein, said. layer of Canada balsam and sheet. of dielectric material :are coextensive with said electrode, said plastic coating overlaps the edges of said sheet of dielectric material and said electrode, and said sheet of dielectric material is non-inflammable.

4. An electroluminescent lamp comprising, in the order named, a radiation-transmitting base member, a radiationtransmitting electrode, a layer comprising electroluminescent phosphor, a thin metal electrode, a layer of Canada balsam, a sheet of dielectric material that is coextensive with said metal electrode, and a layer of cured epoxy resin bonded to said sheet of dielectric material- 5. An electroluminescent lamp as set forth in claim 4 wherein, said base member comprises a glass plate, said sheet of dielectric material comprises a sheet of mica,

References Cited in the file of this patent UNITED STATES PATENTS Knochel et a1 Apr. 17, 1962 OTHER REFERENCES Destriau: The New Phenomenon of Electrophotoluminescence, Philosophical Magazine Ser. 7, vol. 38, No.. 285, pp. 700-737, October 1947. 

1. IN AN ELECTROLUMINSCENT DEVICE, THE COMBINATION OF AN ELECTRODE THAT IS SUSCEPTIBLE TO DAMAGE FROM ELECTRIC ARCS, A LAYER OF CANADA BALSAM ON THE OUTER SURFACE OF SAID ELECTRODE, A SHEET OF DIELECTRIC MATERIAL OVER SAID LAYER OF CANADA BALSAM. AND A COATING OF CURED THERMOSETTING PLASTIC COVERING SAID SHEET OF DIELECTIC MATERIAL. 